Quoting from ADA:

"After her death in 1883, Van computed that in the course of thirteen years, counting every presumed moment of presence, counting the dismal visits to her various hospitals, as well as her sudden tumultuous appearances in the middle of the night (wrestling with her husband or the frail but agile English governess all the way upstairs, wildly welcomed by the old appenzeller — and finally making the nursery, wigless, slipperless, with bloodied fingernails), he had actually seen her, or been near her, all in all, for a length of time hardly exceeding that of human gestation."

Concentrating on Appenzeller we may find in this first chapter of Ada a reference to TT.
The dog that aids Hugh´s blind memory and that belonged to a "vegetable/ appleseller" could be the same "appenzeller dog" of Van´s remembrance of Aqua.

TT Chapter 12: " Person´s power of orientation failed him but a woman selling apples from a neighboring stall set him straight again. An overaffectionate large white dog started to frisk unpleasantly in his wake and was called back by the woman"

TT Chapter 24 ( in fine) " Just beyond it a woman was selling vegetables from a stall (...) As she spoke, a large, white, shivering dog crawled from behind a crate and with a shock of tulite recognition Hugh remembered that eight years ago he had stopped right here and had noticed that dog, which was pretty old even then and had now braved fabulous age to serve his blind memory"